This application relates to electric capacitors in general and, in particular, to an improved dielectric fluid for capacitors.
In the past, polychlorinated biphenyls were widely used as an impregnating liquid in electrical devices including power capacitors. The liquid served as an insulating medium to prevent partial breakdown in the spaces between the solid members of the dielectric. If these spaces were not filled with a good dielectric material, partial discharges would occur under electrical stress and the device would soon fail. Environmental problems with the polychlorinated biphenyls, however, led to a search for alternatives.
More recent capacitor dielectric fluids include such materials as isopropyl biphenyl as disclosed in U.S. Pat. No. 4,054,937, or phenyl xylyl ethane. Other dielectric fluids which have been disclosed as suitable for use as a capacitor dielectric include a mixture of methyl diphenyl ethane and lower alkyl diphenyl as disclosed in U.S. Pat. No. 4,320,034 and mono and dibenzyltoluene(phenyl tolyl methanes) as disclosed in Japanese Patent Specification 55-5689. Isopropyl biphenyl is currently used by one U.S. manufacturer of power factor correction capacitors, phenyl xylyl ethane by another and a third manufacturer uses a mixture of both of these two fluids.
While the above dielectric fluids operate satisfactorily, it is desirable to have a dielectric with increased partial discharge inception voltage. This would allow the operating stress of the capacitor to be increased because of improved partial discharge characters. This in turn reduces the amount of materials required to make the device and, as a result, reduces the cost of the device.